Innovative Bridge Design Enables Defence Force to Access New Training Area

An innovative structural design developed by leading consulting firm, Sinclair Knight Merz, was a key factor in Steelcon Constructions’ award-winning success in building the Bradshaw Bridge across the Northern Territory’s challenging Victoria River.

Completed in September this year, the A$8.9 million bridge won the Northern Territory Construction Association’s award for Best Civil Division Project of 2002, and was also the outright winner of the Association’s Outstanding Project of the Year Award.

Its completion provides access for the Australian Army’s 1st Brigade and other Australian Defence Force units to move onto the new and previously almost inaccessible Bradshaw Field Training Area (BFTA).

The BFTA is a former cattle property known as Bradshaw Station, which was purchased by the Australian Department of Defence in 1996 for use as a live fire combat training facility.

The area is located about 600 kilometres southwest of Darwin and covers more than 8700 square kilometres.

It is bounded to the south by the Victoria River, which separates it from the Victoria Highway, the only sealed all-weather access road in the vicinity.

The Department of Defence commissioned the Construction Division of the Northern Territory’s Department of Infrastructure, Planning and Environment (DIPE) to carry out the procurement, project management and construction supervision of the Bradshaw Bridge and its associated access road.

In response to the DIPE’s call for tenders, Darwin-based Steelcon Constructions engaged Sinclair Knight Merz to develop an alternative to the bridge design originally proposed in the tender documents.

The submission of alternative designs is a standard feature of the Northern Territory Government’s tendering process.

Sinclair Knight Merz Project Design Manager, Ross Dunkley, said that the new design developed by his Structural Team was innovative yet simple, robust enough to handle the challenges posed by the River, and aimed at delivering cost savings to the DIPE and Department of Defence.

"At the height of the wet season, the Victoria River’s main flow has a peak velocity of up to six metres per second and carries a large quantity of logs and debris, all of which put significant lateral forces onto a bridge’s structure," Mr Dunkley said.

"This meant we had to devise a substructure that would ensure the bridge is strong enough to withstand the River’s forces, yet be relatively quick and inexpensive to build," he said.

The Sinclair Knight Merz design resulted in the construction of a 270-metre long submersible bridge comprising twelve 22.5-metre spans supported on bipod piers, capped with a prestressed concrete plank deck that sits 13 metres above the riverbed.

Each bipod pier consists of two 1200-mm diameter piles that rake in towards the headstock and are set into steel-lined sockets drilled into the rock beneath the riverbed.

The upstream piles are set 3.6 metres into the rock to provide sufficient tension anchorage against the forces of the River, and the downstream piles are set one metre into the rock, providing a solid foundation anchorage.

Consultants from the Darwin office of Douglas Partners provided geotechnical and rock mechanics expertise to assist Sinclair Knight Merz in designing the rock sockets.

"Because we specified the tops of the steel socket liners to be at a higher level than the dry season river flow, the construction crew was able to continue working through the early part of the wet season as the River rose," Mr Dunkley said.

"The simplicity and strength of the design also enabled us to reduce the original number of bridge spans," he said.

"Together, these factors produced cost savings of around 10%," Mr Dunkley added.

Prior to the construction of the Bradshaw Bridge and its access road, the only way into the BFTA was either by barge across the Victoria River from the south, by four-wheel drive across the river at low tide or via a land-track from the northeast – both only traversable during the dry season – or by air.

Now, the Australian Defence Force has all-weather road access to the training field during all but a few days of each year at the height of the monsoonal wet season, when the Bridge may become completely submersed.

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